The present invention concerns a device for starting an internal combustion engine as defined in the preamble to the main claim.
Such devices conventionally include an electric machine associated with a transmission shaft provided with a toothed member, or pinion. During starting, the pinion is brought into engagement with a corresponding toothed member, or ring, of the internal combustion engine. On engagement, the torque generated by the electric machine is transferred to the shaft of the internal combustion engine.
Various arrangements are known in the art for the production of the simplest and least cumbersome starter motors possible.
In almost all cases, a specific two-phase operative arrangement is now preferred: a first phase in which the pinion is engaged with the toothed ring in order to couple the starter motor and the internal combustion engine; and a second phase in which the starter motor is supplied at full current and torque transfer takes place.
Arrangements are known in which the pinion is engaged with the ring before the electric machine is started, for example, by means of a lever controlled by the displacement of the movable core of an electromagnet disposed alongside the electric machine.
Arrangements are also known in which the coupling of the pinion with the ring occurs simultaneously with the starting of the electric machine and is, in fact, caused by the particular mode of operation of this latter.
In particular, an arrangement is known in which the rotor of the electric machine is disposed, at rest, partially offset in a longitudinal direction with respect to the stator.
The electric machine is so formed that, when it is energised, a magnetic attraction is generated between the stator and the rotor which causes the axial translation of this latter towards the stator. The translating rotor accompanies the pinion to engage the corresponding toothed ring.
During operation, the electrical supply to the electric machine is controlled by an electromagnetic switch.
During the first phase, the switch is arranged to supply a limited current to an auxiliary stator winding of the electric machine, sufficient to cause the attraction and translation of the rotor (together with a slow rotation), and the coupling of the pinion with the ring.
During the second phase, the switch supplies the principal stator winding of the electric machine with a high current in order to deliver a high torque.
A disadvantage with these arrangements is that they require the presence of an electromagnetic switch which is generally mounted such that it projects, thus increasing the bulk of the starter motor.
Further problems arise during the assembly of the entire device on the ring of the engine intended to receive it: the presence of the switch impedes the formation of a device having substantially axial symmetry, and restricts its spatial orientation. This gives rise to the necessity of producing different elements to support and fix the device depending on the particular engine for which it is intended, with a consequent unwanted increase in production time and costs.
A further disadvantage is the excessive complexity of known starter motors, both from the mechanical point of view, due to the presence of complicated kinematic mechanisms for the translation, engagement and disengagement of the pinion, and from the electrical point of view, as two distinct field windings are sometimes required for the stator.